This invention relates to the field of interferometry, and particularly to scanning interferometers intended for use in spectrometry. More specifically, its primary focus is on improving Michelson interferometers intended for use in infrared Fourier transform spectroscopy.
The present invention is an improvement relating to the same general subject matter as three earlier filed applications assigned to the assignee of this application. Two of those applications were filed by the same inventor as the present application. The earliest was Doyle application Ser. No. 790,457, filed Apr. 25, 1977, now U.S. Pat. No. 4,190,366 titled "Refractively Scanned Interferometer." That patent discloses and claims an interferometer having stationary reflectors at the end of each interferometer "arm" combined with a wedge-shaped prism movable across one arm to cause scanning.
The other of the earlier related Doyle applications is Ser. No. 808,951, filed June 22, 1977, also titled "Refractively Scanned Interferometer", and issued on Aug. 28, 1979 as U.S. Pat. No. 4,165,938. That patent discloses and claims an interferometer of the same type as Application Ser. No. 790,457, now U.S. Pat. No. 4,190,366 wherein the orientation and direction of motion of the wedge-shaped scanning prism are such that displacement of the refracted optical beam by the prism is minimized or eliminated.
The third earlier-filed, commonly assigned patent application relevant to the present subject matter is Ser. No. 61,010, filed July 26, 1979 by William L. Clarke. The present application is directed to the same general purpose as Ser. No. 61,010, but is a significant improvement thereover, because it obtains the same or greater functional advantages with substantially lower cost and better operating efficiency.
As pointed out in Clarke application Ser. No. 61,010, the interferometers shown in the earlier Doyle applications have provided a significant development in the field of Fourier transform spectroscopy. However, certain performance limitations have become apparent in those interferometers, which are attributable to certain optical aberrations not fully compensated for in the earlier Doyle interferometers.
There are three general types of optical aberrations encountered in apparatus of this type: (a) chromatic aberration, (b) spherical aberration, and (c) astigmatic aberration.
The Clarke application Ser. No. 61,010 discusses at length the respects in which the earlier Doyle apparatus was subject to these optical aberrations. It is not necessary to repeat that discussion here.
The Clarke application provides an effective functional solution to these aberration problems by using a fully symmetrical interferometer, i.e., an interferometer in which the two interferometer arms are "mirror images" of one another when the movable prism is in its centered position.
In the Clarke apparatus, this symmetrical, or balanced, arrangement requires the use of two additional optical elements--one, an element matching, or balancing, the optical refractive effect of the beamsplitter substrate, and the other, a wedge-shaped prism matching, or balancing, the refractive effect of the movable prism in its centered position. In each case the added balancing means is located in the other arm of the interferometer from the member whose refractive effect it balances.
Thus the Clarke apparatus solves the aberration problems quite effectively, but it does add two optical elements to the interferometer. The adding of these elements has a two-fold disadvantage. They add substantial cost to the interferometer structure; and they cause significantly larger optical losses due to the added reflecting surfaces. In the Clarke apparatus, the larger optical losses are due both (a) to the simple reflection losses resulting from the added reflecting surfaces, each of which causes a loss of approximately 4% of the radiation, and (b) to the degradation in coherence resulting from any imperfections in these added surfaces. The latter effect can be significant in view of the difficulty of polishing otherwise desirable optical materials, such as potassium bromide (KBR), which is desirable because of its broad optical characteristics, i.e., the broad infrared transmission it permits.
The purpose of the present invention is to provide a structure which retains the aberration-compensating benefits of the Clarke apparatus, while eliminating the disadvantages of increased cost and optical losses, mentioned in the preceding paragraph.